This invention relates to the field of pulse-width modulation (PWM) systems which cause currents to flow through a load in response to an input signal. The invention is particularly useful in the field of motion control, such as in servo-amplifiers, brushless motors, and in other similar applications.
It has been known to control a motor with a servomechanism, wherein the current through the motor is held at a desired value, the desired value being represented by a command signal. The servomechanism regulates the current in the motor by comparing the command signal with a feedback signal, the latter being an appropriately scaled signal representative of the actual motor current. The signal representing the difference between the command signal and the feedback signal is called the "error signal", and this error signal is used to drive an amplifier which applies current to the load.
Linear power amplifiers have been used for increasing the level of the error signal, so as to provide a signal capable of driving the motor. However, linear amplifiers dissipate power, and this power dissipation substantially reduces the efficiency of the system. A system which dissipates power must be provided with heat sinks, cooling fans, and similar apparatus, and the system's size and weight is therefore increased. The energy used to develop the power dissipated in the amplifier is wasted, increasing the overall cost of operation. Furthermore, excessive heat is known to shorten the useful lives of the semiconductor devices used in the amplifier.
Because of the above-described disadvantages of the linear amplifiers used to drive motors, it is clearly preferable to apply current to the load in a manner which itself does not dissipate power. A pulse-width modulated (PWM) circuit approaches this goal. In a pulse-width modulated circuit, the input signal representing the current to be applied to the load is used to generate a train of pulses, the width of each pulse being related to the instantaneous value of the input signal. The pulses are generated by using a comparator to compare the current signal with a dither signal, which is a sawtooth or triangular wave. When the input signal exceeds the dither signal, the output of the comparator is high; at other times, the output of the comparator is low. The comparator output thus comprises the train of pulses representing the input signal.
The pulses are then used to drive an electronic switching device, such as one or more transistors, for intermittently applying a voltage across the load. When transistors are used as switches, they are either fully on (i.e. saturated) or fully off ("cut-off"). Thus, virtually no power is dissipated in the transistors, because when the transistors are saturated, there is almost no voltage drop, and when they are cut-off, there is negligible current flow. Thus, in effect, a PWM system includes an electronic switch, or set of switches, for applying the voltage of the power supply across the load, wherein the switches do not themselves consume appreciable power.
In practice, transistor switches do consume small amounts of power, because they are never totally cut-off or resistance-free. But the efficiency of a PWM circuit can be as high as about 90-95%, compared with only about 40% for linear amplifiers.
Examples of PWM circuits appear in U.S. Pat. Nos. 5,070,292 and 5,081,409. This specification incorporates by reference the disclosures of the latter patents. In the circuits described in the latter patents, and in other PWM circuits of the prior art, a stream of pulses controls electronic switches which open and close different circuit paths for applying current to the load. The widths of the pulses determine when, and in what direction, the circuit applies current to the load. Thus, the pulse widths directly control the effective current in the load.
The present invention provides a PWM circuit and method which is especially suited for use in a digital implementation of a PWM technique. The invention provides a means for sampling a current signal at times which maximize the reliability of the resulting PWM signal.